95 kenworth t800 fuel transferring to passenger side

So I was thinking . What if I plug the return line on passenger side and see if it gets any lower . If it does it means the suction side is good right ? And if it doesn't I'm guessing there's restriction on the return line on the one that's getting empty

 

Well it's still running with the return line on passenger plugged. I'm thinking it's suction problem on the passenger tank

 

Yes I did they both fill up the same . And I don't have shut off valves on tank . This is my setup. So it has 2 lines each tank on top . One is the return line the other is feeder. These lines meet up behind cab . On the return lines it goes to a splitter t valve then to engine . The feeder lines both go to a regular t valve then to engine .pretty simple. So since the return flow is okay I'm thinking it can be the feeder line from t fitting to the passenger side tank all the way down to the tube inside tank . Because first of all for was dry when I loosened it and second my truck wouldn't be running right now . So it's sucking fuel from driver side(the one that's getting empty)

 

It sucks it and returns it evenly from both tanks

 

So I think I might of fixed promblem. So passenger side tank had a bottle cap right underneath suction tube . Removed it and it seems to be sucking fuel . Loosened up fitting and started getting wet . I'll keep ya updated after this shift

 

Thanks for everyone that helped out. This wwbsite is bad ###. Helps me out help you guys out for future reference . So my truck is good now. Works like it should now. So my main problem was when I checked the feeder tube to see if it was clogged I blew air in threw top and I can hear bubbles so I figured it was not clogged but when suction happens, it sucks the bottle cap back in not letting fuel in. Found this by actually taking off tube and noticing cap right under it